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<a name="Writing-JIT-Debug-Info-Readers"></a>
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<a name="Writing-JIT-Debug-Info-Readers-1"></a>
<h4 class="subsection">29.4.2 Writing JIT Debug Info Readers</h4>
<a name="index-writing-JIT-debug-info-readers"></a>

<p>As mentioned, a reader is essentially a shared object conforming to a
certain ABI.  This ABI is described in <samp>jit-reader.h</samp>.
</p>
<p><samp>jit-reader.h</samp> defines the structures, macros and functions
required to write a reader.  It is installed (along with
<small>GDB</small>), in <samp><var>includedir</var>/gdb</samp> where <var>includedir</var> is
the system include directory.
</p>
<p>Readers need to be released under a GPL compatible license.  A reader
can be declared as released under such a license by placing the macro
<code>GDB_DECLARE_GPL_COMPATIBLE_READER</code> in a source file.
</p>
<p>The entry point for readers is the symbol <code>gdb_init_reader</code>,
which is expected to be a function with the prototype
</p>
<a name="index-gdb_005finit_005freader"></a>
<div class="smallexample">
<pre class="smallexample">extern struct gdb_reader_funcs *gdb_init_reader (void);
</pre></div>

<a name="index-struct-gdb_005freader_005ffuncs"></a>

<p><code>struct gdb_reader_funcs</code> contains a set of pointers to callback
functions.  These functions are executed to read the debug info
generated by the JIT compiler (<code>read</code>), to unwind stack frames
(<code>unwind</code>) and to create canonical frame IDs
(<code>get_Frame_id</code>).  It also has a callback that is called when the
reader is being unloaded (<code>destroy</code>).  The struct looks like this
</p>
<div class="smallexample">
<pre class="smallexample">struct gdb_reader_funcs
{
  /* Must be set to GDB_READER_INTERFACE_VERSION.  */
  int reader_version;

  /* For use by the reader.  */
  void *priv_data;

  gdb_read_debug_info *read;
  gdb_unwind_frame *unwind;
  gdb_get_frame_id *get_frame_id;
  gdb_destroy_reader *destroy;
};
</pre></div>

<a name="index-struct-gdb_005fsymbol_005fcallbacks"></a>
<a name="index-struct-gdb_005funwind_005fcallbacks"></a>

<p>The callbacks are provided with another set of callbacks by
<small>GDB</small> to do their job.  For <code>read</code>, these callbacks are
passed in a <code>struct gdb_symbol_callbacks</code> and for <code>unwind</code>
and <code>get_frame_id</code>, in a <code>struct gdb_unwind_callbacks</code>.
<code>struct gdb_symbol_callbacks</code> has callbacks to create new object
files and new symbol tables inside those object files.  <code>struct
gdb_unwind_callbacks</code> has callbacks to read registers off the current
frame and to write out the values of the registers in the previous
frame.  Both have a callback (<code>target_read</code>) to read bytes off the
target&rsquo;s address space.
</p>
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